Fuel cell system and method of controlling the same

ABSTRACT

In a fuel cell system and its controlling method, the fuel cell system includes a stack  21  including fuel cells  11  each having a polymer electrolyte membrane  13 , and a controller  61 . The controller  61  is responsive to detected outputs of a displacement sensor  27  and a temperature sensor  27  and controls such that, when the polymer electrolyte membrane  13  is discriminated to remain in an excessively dry state, a shut-off valve  37  is applied with a “close” control signal to interrupt the supply of fuel gas to the stack  21  and, concurrently, a shut-off valve  41  is applied with an “open” control signal to allow air to be supplied to the stack  21  while applying a pump control signal to a pump  57  so as to maximize its rotational speed for thereby increasing the flow rate of pure water  59  to be circulated to the humidifier  35  from a pure water tank  55 . Simultaneously, a timer of the controller  61  is operated to begin counting an incremental time. As a result, air is excessively humidified by the humidifier  35  and is supplied to the stack  21  via the shut-off valve  41.

TECHNICAL FIELD

The present invention relates to a fuel cell system and a method ofcontrolling the same, and more particularly, to a fuel cell controlsystem of a polymer electrolyte type and a method of controlling a fuelcell of the polymer electrolyte type.

BACKGROUND ART

In recent years, considerable research and development work has beenundertaken to commercially apply a fuel cell of polymer electrolytemembrane type, which has a high power density and which can be operatedat low temperature, as an electric power generation system in a motorvehicle.

Such a fuel cell of the polymer electrolyte membrane is usuallyconstructed of a polymer electrolyte membrane, an anode joined to oneside of the membrane, and a cathode joined to the other side of themembrane to provide a joined structure, which is sandwiched betweenseparators.

In a fuel cell system including a plurality of fuel cells of polymerelectrolyte membrane type as a stack, fuel gas and air are usuallyhumidified by a humidifier for preventing the polymer electrolytemembrane from being dried such that the polymer electrolyte membrane iskept in a suitably wet state throughout electric power generation.

DISCLOSURE OF INVENTION

In such a fuel cell system, however, a usual practice normally done inusual operation of the stack has been adapted to start up the stack,even when the fuel cell system is left in a non-use state for a longtime period and the polymer electrolyte membrane remains in anexcessively dry state.

For this reason, humidification of the polymer electrolyte membrane to asufficiently wet state needs a preliminary longer operating time to someextent, with a resultant difficulty in taking out electric power outputfrom the stack in a stable fashion or an undesirable system failureowing to rapid voltage drop caused when a large amount of electric poweroutput is required.

The present invention has been made in view of the above-describedstudies and has an object to provide a fuel cell system and a methodcontrolling the same which can achieve a substantially optimum start-upcontrol even when a fuel cell system of a polymer electrolyte typeremains in a dry state.

A fuel cell system of the present invention is provided with: ahumidifier humidifying fuel gas and air; a stack producing electricpower by reacting the fuel gas humidified by the humidifier and the airhumidified by the humidifier, including a plurality of fuel cells andfixed for free movement in a stacked direction thereof, each of theplurality of fuel cells having a polymer electrolyte membrane; adisplacement sensor detecting a displacement value in length of thestack in the stacked direction; a temperature sensor detecting atemperature of the stack; and a controller discriminating whether thepolymer electrolyte membrane is in a dry state in response to thedisplacement value of the stack detected by the displacement sensor andthe temperature of the stack detected by the temperature sensor andcontrolling the humidifier, when the polymer electrolyte membrane isdiscriminated as in a dry state at a start of operation of the stack, tocause the polymer electrolyte membrane to be brought into a wet state.

In other words, a fuel cell system of the present invention is providedwith: humidifying means humidifying fuel gas and air; a stack producingelectric power by reacting the fuel gas humidified by the humidifyingmeans and the air humidified by the humidifying means, including aplurality of fuel cells and fixed for free movement in a stackeddirection thereof, each of the plurality of fuel cells having a polymerelectrolyte membrane; displacement detecting means detecting adisplacement value in length of the stack in the stacked direction;temperature detecting means detecting a temperature of the stack; andcontrolling means discriminating whether the polymer electrolytemembrane is in a dry state in response to the displacement value of thestack detected by the displacement detecting means and the temperatureof the stack detected by the temperature detecting means and controllingthe humidifying means, when the polymer electrolyte membrane isdiscriminated as in a dry state at a start of operation of the stack, tocause the polymer electrolyte membrane to be brought into a wet state.

Besides, a method of controlling a fuel cell system is applied to asystem provided with a humidifier humidifying fuel gas and air, and astack producing electric power by reacting the fuel gas humidified bythe humidifier and the air humidified by the humidifier, including aplurality of fuel cells and fixed for free movement in a stackeddirection thereof, each of the plurality of fuel cells having a polymerelectrolyte membrane. The method detects a displacement value in lengthof the stack in the stacked direction and a temperature of the stack,discriminates whether the polymer electrolyte membrane is in a dry statein response to the displacement value of the stack detected by thedisplacement sensor and the temperature of the stack detected by thetemperature sensor, and controls the humidifier, when the polymerelectrolyte membrane is discriminated as in a dry state at a start ofoperation of the stack, to cause the polymer electrolyte membrane to bebrought into a wet state.

Other and further features, advantages, and benefits of the presentinvention will become more apparent from the following description takenin conjunction with the following drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view for illustrating a cell structure of a polymerelectrolyte membrane type fuel cell to be incorporated in a fuel cellsystem according to a first preferred embodiment of the presentinvention;

FIG. 2 is a schematic view for illustrating a typical example of afixing structure for a stack constructed of a plurality of fuel cellsaccording to the embodiment;

FIG. 3 is a view for illustrating a displacement characteristic inlength of the stack in a stacked direction depending on differences in awet state of the stack according to the embodiment;

FIG. 4 is a block diagram of a starter device for a fuel cell systemaccording to the embodiment;

FIG. 5 is a flow diagram to illustrate the operation of the starterdevice of the fuel cell system according to the embodiment;

FIG. 6 is a block diagram of a starter device of a fuel cell systemaccording to a second preferred embodiment of the present invention; and

FIG. 7 is a flow diagram to illustrate the operation of the starterdevice of the fuel cell system according to the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

To describe the present invention in more detail, preferred embodimentsof the present invention will be explained with reference to thedrawings below.

(First Embodiment)

FIG. 1 shows a cell structure of a polymer electrolyte type fuel cellfor a fuel cell system of a preferred embodiment according to thepresent invention.

In FIG. 1, the fuel cell 11 is constructed having a polymer electrolyte13, an anode electrode 15 located on one side of the polymer electrolytemembrane 13, a cathode 17 located on the other side of the polymerelectrolyte membrane 13 to form a membrane electrode assembly (MEA),which is sandwiched between separators 19.

FIG. 2 is a view illustrating a typical example of a fixing structure ofa stack which is comprised of a plurality of fuel cells.

Since the stack 21 is constructed of a large number of fuel cells 11,the stack 21 causes the polymer electrolyte membrane 13 to swell due tohumidifying water, with a resultant expansion and contraction caused ineach of the separators 19 in a stacked direction X owing to thermalexpansion.

For this reason, one distal end of the stack 21 is fixedly fastened to avehicle body 25 by means of a stationary fixture 23, and the otherdistal end of the stack 21 is connected to the vehicle body 25 via amovable fixture 24 so as to prevent vertical movement in a direction Zwhile allowing expanding and contracting movement in the stackeddirection X. Also, a displacement sensor 27 is located on the movablefixture 24 and detects current displacement value relative to anoriginal length of the stack 21 measured at a fabricating stage thereof,producing a displacement detection signal 27 a which is delivered to acontroller 61 in a manner as will be described in detail.

FIG. 3 is a view for illustrating the displacement characteristic of thestack 21 in conjunction with a varying length in the stacked direction Xdue to a difference in a wet state of the stack 21.

In the event the polymer electrolyte membrane 13 of the stack 21 doesnot contain water immediately when it has been fabricated or when thestack 21 has been left in anon-operating state for a long time period tocause the stack 21 to remain in an extremely dry state, the stack 21remains in a minimum displacement position P1, representing the shortestlength of the stack and an extremely dry state, as shown in FIG. 3.

On the contrary, when the stack 21 is continuously humidified in asuitably wet state and is operating at the maximum power output, thestack 21 encounters swelling and thermal expansion to assume a properwet position P5, equal to the longest length of the stack, as shown inFIG. 3 during electric power generation. Further, when the operation ofthe fuel stack 21 is repeated at suitable periods, the polymerelectrolyte membrane 13 is liable to contain water and, therefore, thestack 21 assumes a normal wet position P3, equal to an intermediatelength closer to the proper wet position P5, as shown in FIG. 3 evenwhen the stack 21 remains in a normal non-operating state.

FIG. 4 is a block diagram showing a starter device 31 of a fuel cellsystem of a first preferred embodiment according to the presentinvention. A structure of the starter device 31 of the fuel cell systemS of FIG. 3 will be mainly described below in detail.

Fuel gas 33 is hydrogen gas or reformed gas delivered from for example areformer (not shown) and is supplied to a humidifier 35 which humidifiesfuel gas 33. Humidified fuel gas is then supplied through a shut-offvalve 37 to the stack 21. Also, air 39 is supplied from for example anair compressor (not shown) and is humidified in the humidifier 35.Humidified air is then supplied through a shut-off valve 41 to the stack21.

In the stack 21, humidified fuel gas and humidified air are reacted toproduce electric power output, with non-reacted exhaust gas 43 beingemitted and non-reacted air 45 being also exhausted through a condenser47 which emits exhaust air 49. Further, a temperature sensor 28 ismounted on the stack 21 for detecting the temperature thereof andproducing a temperature detection signal 28 a which is applied to thecontroller 61.

In the condenser 47, air 45 emitted from the stack 21 is passed throughplural cooling fins through which coolant 51 is circulated, therebycondensing surplus water contained in air 45 to recover the same.Recovered pure water 53 is fed to a pure water tank 55.

The pure water tank 55 serves to capture ions from pure water 53 withthe use of an ion filter and stores the same therein and, subsequently,pure water 59 is pressurized with a pump 57 and supplied to thehumidifier 35.

The controller 61 includes a RAM (not shown) which stores control data,a ROM (not shown) which stores control programs, a CPU (not shown) whichcontrols a system in accordance with the control programs, and a timer(not shown) which counts preset increment times to produce aninterrupting signal INT which is applied to the CPU.

The controller 61 is responsive to the displacement detection signal 27a delivered from the displacement sensor 27 and the temperaturedetection signal 28 a delivered from the temperature detection sensor 28to discriminate whether the polymer electrolyte membrane located in thestack 21 remains in a dry state or is in a wet state, thereby producinga pump control signal 61 a so as to control the shut-off valves 37 and41 and the pump 57 such that, in the event of an excessively dry state,they are operated in a dry-state start sequence and, in the event of thewet state, they are switched over to and are controlled to startoperation in a normal start sequence.

A DC/DC converter 63 is applied with electric power output from thestack 21 and functions to achieve a voltage boost conversion or voltagelowering conversion responsive to a demanded power command signal 61 bdelivered from the controller 61 for thereby controlling and limitingthe amount of electric power output, to be delivered from the stack 21,which is supplied through the DC/DC converter 63 to a battery (notshown) which serves as a load, and other load units (not shown).

Now, the operation of the starter device 31 of the fuel cell system ofthe first preferred embodiment is described below in detail withreference to a general flow diagram shown in FIG. 5. The basic sequenceof operations illustrated in the general flow diagram of FIG. 5 isstored as the control program in the internal ROM of the controller 61.

At the start-up operation, power is applied to the starter device 31 ofthe fuel cell system, thereby starting up operation of the controller61. At this instant, the controller 61 begins to read out the controlprogram stored in the ROM and to control in a manner described below.

In a first execution of step S10, the displacement value L in the lengthof the stack 21 and the temperature T thereof are watched and detectedat all times by the displacement sensor 27 and the temperature sensor28, respectively.

In conjunction with the displacement value L of the stack 21, the lengthof the stack 21, which has been originally measured at the fabricationstage, is preliminarily stored as a reference data in the RAM of thecontroller 61. Also, when the stack 21 is replaced with new one, it isrequired to renew this reference data. Further, the RAM of thecontroller 61 preliminarily stores therein parameters covering otherdisplacement values caused by thermal expansion responsive to the stacktemperature T.

In the next step S20, the wet state of the polymer electrolyte membraneis discriminated in terms of the displacement value L and thetemperature T which are detected by the displacement sensor 27 and thetemperature sensor 28, respectively. A process for discriminating thewet state of the polymer electrolyte membrane will be described indetail below.

For instance, in a case where a separator is made of for example carbon,the instantaneous value is obtained from the original length A measuredat the fabricating stage, the length B measured when the stack is heldin a non-operating state under a suitable wet state (namely, when thestack is held in the non-operating state at a normal temperature), thelength C measured when the stack is operating in a normal state at themaximum power output, and the maximum displacement value d, caused bythermal expansion, of the stack, i.e. is expressed by the followingrelation:

 A+d<B−d  (1)

Also, the maximum displacement valued of the stack caused by thermalexpansion is derived from the coefficient β of linear expansion in thestacked direction X, the temperature T and the length A′ measured at 0°C. during the fabricating step and is expressed as:

d=β×T×A′  (2)

With this equation (2), the formula (1) is expressed as:

A+β×T×A′<B−β×T×A′  (3)

The formula (1) is proved from a reason in that although the coefficientof thermal expansion of the separator made of carbon is on the order of10⁻⁶, the amount of swelling (,i.e. though it depends on the filmthickness and a chemical structure of the polymer electrolyte membrane)of the polymer electrolyte membrane due to wet state is on the order of10⁻¹.

For this reason, the stack encounters thermal expansion which is largerin value at all times when the stack temperature is low under a suitablewet state in the polymer electrolyte membrane, than that encounteredwhen the stack temperature is high under an extremely dry state in thepolymer electrolyte membrane.

Accordingly, in the event the length L of the stack detected by thedisplacement sensor 27 at the start of operation is obtained by:

L<B−d=B−β×T×A′  (4)

that is, when the length L of the stack is less than the instantaneousvalue in which the maximum displacement value d caused by thermalexpansion is subtracted from the length B of the stack measured when itis held in the non-operating state under the suitable wet state, it canbe found that the polymer electrolyte membrane remains in theexcessively dry state.

In the second execution of step S20, also, the controller 61discriminates on the basis of the detected values of the displacementsensor 27 and the temperature sensor 28 that the polymer electrolytemembrane remains in the excessively dry state and, in this event, theoperation goes to step S30.

In step S30, the shut-off valve 37 located between the humidifier 35 andthe stack 21 is applied with a “close” control signal for interruptingthe supply of fuel gas to the stack 21 and, concurrently, the shut-offvalve 41 located between the humidifier 35 and the stack 21 is appliedwith an “open” control signal for allowing air to be supplied to thestack 21. Simultaneously, the pump 57 is applied with the pump controlsignal which allows the rotational speed to be maximized, therebyincreasing the flow rate of pure water 59 to be circulated to thehumidifier 35 from the pure water tank 55. At the same time, the timerin the controller 61 begins to count the incremental time. As a result,air, which is excessively humidified by the humidifier 35, is suppliedto the stack 21 through the shut-off valve 41.

In the succeeding step S40, the counted incremental time is read outfrom the timer and the polymer electrolyte membrane of the stack 21 ishumidified until the humidifying time period reaches a predeterminedtime period Sh.

When the humidifying time period reaches the predetermined incrementaltime Sh, the operation goes to step S50 and, in this step, in additionto supplying the air, the shut-off valve 37 located between thehumidifier 35 and the stack 21 is applied with an “open” control signalto begin the supply of fuel gas to the stack 21 while applying the pumpcontrol signal 61 a to the pump 57 such that the flow rate thereof isreturned to its normal value to allow the stack 21 to begin powergeneration.

Then in step S60, in the same manner as previously noted in step S10,the displacement sensor 27 and the temperature sensor 28 detect thedisplacement value L of the length of the stack and the temperaturethereof.

In the succeeding step S70, in the same manner as previously noted instep S20, the controller 61 responds to the detected values of thedisplacement sensor 27 and the temperature sensor 28, respectively, andif it is found that the polymer electrolyte membrane remains in the drystate, the operation returns to step S80.

In step S80, the amount of electric power output to be extracted fromthe stack 21 is regulated to a limited value below a predetermined valuePh. In particular, the electric power output of the stack 21 isdelivered to the DC/DC converter 63, which converts the voltage upwardor downward in response to the demanded power command signal 61 bapplied from the controller 61 and controls the amount of electric powerto be extracted from the stack 21 to the limited value below thepredetermined value Ph, thereby allowing electric power output from theDC/DC converter 63 to the battery (not shown) and the other load units(not shown). Operation then returns to step S60 and the above discussedprocess is repeated.

In the execution of step S70, if it is found that the polymerelectrolyte membrane is not held in the dry state and remains in asufficiently wet state, operation goes to step S90 to allow the stack 21to produce electric power output in a normal operating mode. Inparticular, the demanded power command signal 61 b applied from thecontroller 61 to the DC/DC converter 63 is modulated to have a normalpower level by which the DC/DC converter 63 controls for relaxing thelimitation in voltage boost conversion or voltage lowering conversion,thereby allowing normal electric power to be supplied from the DC/DCconverter 63 to the battery (not shown) and the other load units (notshown).

Thus, it can be found on the basis of the length L of the stack and thetemperature T thereof that the polymer electrolyte membrane located inthe stack 21 is sufficiently wet, and the amount of electric power to beutilized is regulated to the limited value below the predetermined valuePh until the maximum electric power output is obtained. As a result, itis possible to suppress undesirable situations such as a sudden stopcaused in a system owing to some reasons such as rapid voltage drop.

In the execution of step S20, if it is found by the controller 61 on thebasis of the detected values of the displacement sensor 27 and thetemperature sensor 28, respectively, that the polymer electrolytemembrane located in the stack 21 is not held in the excessively drystate and remains in the wet state, operation goes to step S100.

In the execution of step S100, both the shut-off valves 37 and 41located between the humidifier 35 and the stack 21 are applied with“open” control signals, allowing fuel gas and air to be supplied to thestack 21. Concurrently, the controller 61 applies the pump controlsignal 61 a to the pump 57 to render the flow rate thereof to have anormal value, allowing the stack 21 to begin to produce electric poweroutput.

In the succeeding step S110, the controller 61 reads out the detectedvalue of the temperature sensor 28 to discriminate whether the stacktemperature T exceeds or is below a preset temperature Td. When thestack temperature is below the preset temperature Td, operation returnsto step S120 wherein, in the same manner as in the step S80, the amountof electric power to be extracted from the stack 21 is limited below thepredetermined value Ph.

In the execution of the step S110, if it is found that the stacktemperature exceeds the preset temperature Td, operation then returns tostep S90 to allow the stack to operate in the normal power generationmode.

According to the first preferred embodiment of the present invention,the wet state of the polymer electrolyte membrane located in the stackis detected and, even when the polymer electrolyte membrane remains inan excessively dry state, the stack can be started up after the wetstate is recovered, enabling electric power output to be extracted fromthe stack in a stable fashion while avoiding the rapid voltage dropcaused during large electric power generation to enable the fuel cellsystem to start-up in a smooth fashion at all times.

Another advantage of the first preferred embodiment resides in that thefuel cell system can be realized by merely incorporating a simplifiedstructure therein to allow a displacement sensor to be mounted in thestack. As a result, further, it is possible to realize the fuel cellsystem by merely adding this start-up process thereto in a relativelyeasy fashion.

In addition, the wet state of the stack can be detected, with aresultant decrease in trouble shooting time required for some troublesoccurred in the stack.

(Second Embodiment)

FIG. 6 is a block diagram for illustrating the structure of a starterdevice 71 of a fuel cell system of a second preferred embodimentaccording to the present invention. Also, the second preferredembodiment has the same basic structure as that of the fuel cell systemof the first preferred embodiment shown in FIG. 4, with like partsbearing like reference numerals as those used in FIG. 4 while detaileddescription of the like parts are omitted for the sake of clarity.

An essential feature of the second preferred embodiment differs fromthat of the first preferred embodiment in that a humidifier 73incorporates therein a heater 75.

The operation of the starter device 71 of the fuel cell system of thesecond preferred embodiment will be described in detail with referenceto a general flow diagram shown in FIG. 7. The basic sequence ofoperations illustrated in the general flow diagram of FIG. 7 is storedas the control program in the internal ROM of the controller 61. Also,the second preferred embodiment has the same basic steps as those in theflow diagram of the first preferred embodiment shown in FIG. 5, withlike steps bearing like reference numerals as those used in FIG. 5 whilea detailed description of the like steps is omitted for the sake ofclarity.

In the execution of step S130, with which the step S30 in the firstpreferred embodiment is replaced, the shut-off valve 37 located betweenthe humidifier 73 and the stack 21 is applied with a “close” controlsignal to interrupt the supply of fuel gas to the stack 21 while,concurrently, applying an “open” control signal to the shut-off valve 41located between the humidifier 73 and the stack 21 to allow air to besupplied to the stack 21. Simultaneously, the heater 75 located in thehumidifier 73 is applied with electric power 61 c, and the pump controlsignal 61 a is applied to the pump 57 from the controller 61 so as tomaximize the rotational speed of the pump 57 for increasing the flowrate of pure water 59 to be circulated to the humidifier 73 from thepure water tank 55. At the same time, the timer in the controller 61 iscaused to begin counting operation of the incremental time.

As a result, even when the polymer electrolyte membrane located in thestack 21 remains in an excessively dry state at the start of operation,the heater 75 in the humidifier 73 is heated to produce a furtherexcessively humidified air, which is supplied through the shut-off valve41 to the stack 21, with a resultant decrease in the operating timerequired for wetting the polymer electrolyte membrane.

In the execution of step S150, with which the step S50 in the firstpreferred embodiment is replaced, the shutoff valve 37 located betweenthe humidifier 35 and the stack 21 is applied with an “open” controlsignal to begin the supply of fuel gas to the stack 21 in addition tosupplying the air, while the heater 75 located in the humidifier 73 isapplied with electric power 61 c and applying the pump control signal 61a to the pump 57 such that the flow rate thereof is returned to itsnormal value to allow the stack 21 to begin power generation.

As a result, even when the polymer electrolyte membrane located in thestack 21 is once humidified during a predetermined time period, theheater 75 in the humidifier 73 is heated to produce a furtherexcessively humidified air, which is supplied through the shut-off valve41 to the stack 21, with a resultant decrease in the operating timerequired for wetting the polymer electrolyte membrane.

In the execution of step S170, with which the step S110 in the firstpreferred embodiment is replaced, the shut-off valves 37 and 41 locatedbetween the humidifier 73 and the stack 21 are applied with “open”control signals to begin the supply of fuel gas and air to the stack 21while applying electric power to the heater 75 located in the humidifier73. Simultaneously, the pump control signal 61 a is delivered from thecontroller 61 to the pump 57 so as to regulate the flow rate to a normalvalue and to allow the stack 21 to begin electric power generation.

As a result, even when the polymer electrolyte membrane located in thestack 21 remains in a normal, wet state at the start of operation, theheater 75 located in the humidifier 73 is heated to provide anexcessively humidified air to the stack 21 via the shut-off valve 41,allowing the polymer electrolyte membrane to be wet in a reduced,operating time period.

A typical advantage of the second preferred embodiment resides in thatan increase in the amount of humidification reduces the wait time forwetting a polymer electrolyte membrane.

In the present invention, the reliability in operation of the fuel cellsystem is greatly enhanced to provide a stable operation in the fuelcell system at all times even when it remains in an excessively drystate. Unlike a structure of a fuel cell system employing ahumidification process which begins to humidify the polymer electrolytemembrane in a normal mode at the start-up operation, the system of thepresent invention has a controller which allows the polymer electrolytemembrane of the stack to be suitably wet in the shortest period at thestart-up operation even when the polymer electrolyte membrane remains inthe excessively dry state. The obvious result is the elimination of waittime during start-up of the system and system failures that wouldotherwise occur when large power output is consumed when the stackremains in the dry state.

Since the dry state of the stack is discriminated in terms of thedisplacement value in length of the stack in the stacked direction andthe temperature of the stack, an optimum start-up control of the fuelcell system is provided with greatly simplified operating process andrapid start-up of the fuel cell system at full load can be achieved in ahighly reliable manner. This optimum start up control can be achievedwith the use of minimum number of component parts without causing aremarkable increase in cost. Thus, the system of the invention can beeasily applied to existing fuel cell systems to improve the operatingperformance.

The controller controls the fuel cell system such that, when the stackremains in the dry state, humidified air is merely supplied to the stackto render the polymer electrolyte membrane to be wet in the shortperiod. This control is achieved in an easy fashion by opening orclosing plural shut-off valves located between the humidifier and thestack.

Further, during start-up of the fuel cell system, when the polymerelectrolyte membrane of the stack remains in the dry state, thecontroller controls the humidifier such that humidified air is firstsupplied to the stack for a predetermined time period and subsequentlyhumidified fuel gas is also supplied to the stack and also controls thestack such that electric power output generated by the stack is limitedbelow a predetermined value until the polymer electrolyte membrane isbrought into a sufficiently wet state. This results in the eliminationof the voltage drops or system failures caused thereby.

Still further, a heater may be incorporated in the humidifier to providefurther excessively humidified fuel gas and air to the stack, reducingthe operating time required for wetting the polymer electrolyte membraneto a sufficient level for thereby enabling the fuel cell system to startup in a short period even when the polymer electrolyte membrane remainsin the dry state.

INDUSTRIAL APPLICABILITY

As described above, in the present invention, a dry state of a stack ina fuel cell system is watched in terms of a displacement value in lengthof the stack and a temperature of the stack, and the amount ofhumidification in fuel gas and air is controlled to allow a polymerelectrolyte membrane of the stack to be suitably wet in a highlyefficient and reliable manner. Therefore, a wide applicability thereofincluding a fuel cell system for a vehicle is expected.

What is claimed is:
 1. A fuel cell system comprising: a humidifierhumidifying fuel gas and air; a stack producing electric power byreacting the fuel gas humidified by the humidifier and the airhumidified by the humidifier, including a plurality of fuel cells andfixed for free movement in a stacked direction thereof, each of theplurality of fuel cells having a polymer electrolyte membrane; adisplacement sensor detecting a displacement value in length of thestack in the stacked direction; a temperature sensor detecting atemperature of the stack; and a controller discriminating whether thepolymer electrolyte membrane is in a dry state in response to thedisplacement value of the stack detected by the displacement sensor andthe temperature of the stack detected by the temperature sensor, andcontrolling the humidifier, when the polymer electrolyte membrane isdiscriminated as in a dry state at a start of operation of the stack, tocause the polymer electrolyte membrane to be brought into a wet state.2. A fuel cell system according to claim 1, wherein the controllercontrols the humidifier, when the polymer electrolyte membrane isdiscriminated as in the dry state at the start of operation, to supplythe air through the humidifier to the stack for a predetermined timeperiod while not supplying the fuel gas through the humidifier to thestack.
 3. A fuel cell system according to claim 2, wherein thecontroller controls the humidifier, after only the air has been suppliedto the stack through the humidifier for the predetermined time period,to supply the fuel gas and the air through the humidifier to the stack,while controlling the stack such that the electric power output from thestack is regulated to a limited value below a predetermined value untilthe polymer electrolyte membrane is determined as in a wet state.
 4. Afuel cell system according to claim 1, wherein the humidifier includes aheater heating the fuel gas and the air, and the stack produces theelectric power by reacting the fuel gas humidified and heated by thehumidifier and the air humidified and heated by the humidifier.
 5. Afuel cell system according to claim 4, wherein the controller controlsthe humidifier, when the polymer electrolyte membrane is discriminatedas in the dry state at the start of operation, to supply the air throughthe humidifier to the stack for a predetermined time period while notsupplying the fuel gas through the humidifier to the stack.
 6. A fuelcell system according to claim 5, wherein the controller controls thehumidifier, after only the air has been supplied to the stack throughthe humidifier for the predetermined time period, to supply the fuel gasand the air through the humidifier to the stack, while controlling thestack such that the electric power output from the stack is regulated toa limited value below a predetermined value until the polymerelectrolyte membrane is determined as in a wet state.
 7. A fuel cellsystem according to claim 1, wherein the controller controls thehumidifier to supply the fuel gas and the air through the humidifier tothe stack when the polymer electrolyte membrane is discriminated as in awet state at the start of operation of the stack.
 8. A fuel cell systemaccording to claim 7, wherein the controller controls the humidifier tosupply the fuel gas and the air through the humidifier to the stack,while controlling the stack such that the electric power output from thestack is regulated to a limited value below a predetermined value untilthe temperature of the stack exceeds a predetermined temperature.
 9. Afuel cell system comprising: humidifying means humidifying fuel gas andair; a stack producing electric power by reacting the fuel gashumidified by the humidifying means and the air humidified by thehumidifying means, including a plurality of fuel cells and fixed forfree movement in a stacked direction thereof, each of the plurality offuel cells having a polymer electrolyte membrane; a displacementdetecting means detecting a displacement value in length of the stack inthe stacked direction; temperature detecting means detecting atemperature of the stack; and controlling means discriminating whetherthe polymer electrolyte membrane is in a dry state in response to thedisplacement value of the stack detected by the displacement detectingmeans and the temperature of the stack detected by the temperaturedetecting means, and controlling the humidifying means, when the polymerelectrolyte membrane is discriminated as in a dry state at a start ofoperation of the stack, to cause the polymer electrolyte membrane to bebrought into a wet state.
 10. A method of controlling a fuel cell systemprovided with a humidifier humidifying fuel gas and air, and a stackproducing electric power by reacting the fuel gas humidified by thehumidifier and the air humidified by the humidifier, including aplurality of fuel cells and fixed for free movement in a stackeddirection thereof, each of the plurality of fuel cells having a polymerelectrolyte membrane, the method comprising: detecting a displacementvalue in length of the stack in the stacked direction; detecting atemperature of the stack; discriminating whether the polymer electrolytemembrane is in a dry state in response to the displacement value of thestack detected by the displacement sensor and the temperature of thestack detected by the temperature sensor; and controlling thehumidifier, when the polymer electrolyte membrane is discriminated as ina dry state at a start of operation of the stack, to cause the polymerelectrolyte membrane to be brought into a wet state.